1. Field of the Invention
The present invention relates to a liquid crystal display panel, particularly to a transmissive or semi-transmissive liquid crystal display panel of the multi-domain vertically aligned (MVA) type having good display quality, where discrimination is suppressed.
2. Background Art
Liquid crystal display devices are generally thin and lightweight and have a low level of power consumption, and are broadly used for various items, from portable terminals to large-sized television sets. The vertically aligned (VA) type of liquid crystal display panel is known to be used in liquid crystal display devices.
In a VA type of liquid crystal display panel 60, as shown in FIG. 5, liquid crystal having negative dielectric anisotropy is filled between a pair of substrates 62, 64, a pixel electrode 61 being arranged on the substrate 62, and a common electrode 63 being arranged on the substrate 64. Alignment films 66, 67 respectively reside on top of the substrates 62, 64 in vertical alignment treatment, while liquid crystal molecules 65 are vertically aligned when no electric field is applied to the pixel electrode 61 and common electrode 63 as shown in FIG. 5A. Polarizing plates 68, 69 are respectively arranged in a cross-nicol state outside the substrates 62, 64.
Then, since the liquid crystal molecules 65 between the substrates are vertically aligned when no electric field is applied between the electrodes 61, 63, linearly polarized transmission light which has passed one polarizing plate directly passes through a liquid crystal layer and is blocked by another polarizing plate to create a dark state, that is, black display. Further, since the liquid crystal molecules 65 between the substrates are roughly aligned horizontally as shown in FIG. 5B when electric field is applied between the electrodes 61, 63, the linearly polarized transmission light that has passed one polarizing plate becomes birefringent to form elliptically polarized transmission light when passing through the liquid crystal layer, and the light passes through another polarizing plate to create a bright state, that is, white display.
In Japanese Laid-Open Patent Publication No. 11-024225, an MVA type of liquid crystal display panel has been introduced where protrusions or grooves are provided in a pixel to form a plurality of domains in one pixel for the purpose of improving further the viewing angle of the VA type of liquid crystal display panel.
The pixel constitution of the conventional MVA type liquid crystal display panel will be described with reference to FIGS. 6 and 7. Note that FIG. 6 is a plane view of a pixel of an MVA type liquid crystal display panel 70, while FIG. 7 is a cross-sectional view taken along VII-VII line of FIG. 6.
Scanning lines 72 and signal lines 73 are wired in a matrix state on a transparent first substrate 71 such as a glass substrate via a gate insulating film 71′. The region surrounded by the scanning lines 72 and the signal lines 73 corresponds to one pixel, a pixel electrode 74 being arranged in the region, and a TFT 75 that serves as a switching device for connection to the pixel electrode 74 is formed at the crossing area between the scanning line 72 and the signal line 73. A part of the pixel electrode 74 overlaps with an adjacent scanning line 72 while an insulating film 71″ is made to lie therebetween, and the overlapped area works as a retention capacitance. A plurality of slits 76 (described later) is formed on the pixel electrode 74. The alignment film 77 covering the pixel electrode 74 is accorded vertical alignment treatment.
A black matrix 79 is formed on a transparent second substrate 78 such as a glass substrate so as to partition each pixel, and a color filter 80 is laid corresponding to each pixel. Color filters 80 in red (R), green (G) and blue (B) are provided for each pixel. A common electrode 81 made of a transparent indium tin oxide (ITO) electrode, for example, is laid on the color filter 80, while protrusions 82 having a predetermined pattern are formed on the common electrode 81, and an alignment film 83 set in vertical alignment treatment covers the common electrode 81 and the protrusions 82.
A liquid crystal layer 84 having negative dielectric anisotropy is arranged to lie between a pair of substrates 71, 78. Then, when an electric field is not generated between the pixel electrode 74 and the common electrode 81, liquid crystal molecules 84′ are restricted by the alignment films 77, 83 and become vertically aligned, and the liquid crystal molecules 84′ become horizontally inclined when an electric field is generated between the pixel electrode 74 and the common electrode 81. At this point, the liquid crystal molecules 84′ are restricted by the slits 76 and the protrusions 82 and become inclined in a predetermined direction, and thus a plurality of domains can be formed in one pixel. Note that FIG. 7 schematically shows the state where electric field is generated between the pixel electrode 74 and the common electrode 81.
Further, a first polarizing plate 85 is arranged outside the first substrate 71, and a second polarizing plate 86 is arranged outside the second substrate 78, with both polarizing plates being set in such manner that their transmission axes become orthogonal to each other. The orientation of the polarizing plates 85, 86 is set based on the relation between the transmission axes and the orientation of liquid crystal molecules 84′ when they are inclined. While the relation between the transmission axes of the polarizing plates 85, 86 and the inclined position of the liquid crystal molecules 84′ will be described later, suffice it to say at this point that the orientation of the plates is set such that the axis of the first polarizing plate 85 matches the extended direction of the scanning line 72 and the axis of the second polarizing plate 86 matches the extended direction of the signal line 73.
Then, since the liquid crystal molecules 84′ are vertically aligned when an electric field is not generated between the pixel electrode 74 and the common electrode 81, linearly polarized transmission light that has passed the first polarizing plate 85 directly passes through the liquid crystal layer 84 in the state of linearly polarized light and is blocked by the second polarizing plate 86 to create black display. Further, since the liquid crystal molecules 84′ become horizontally inclined when voltage of a predetermined amount is applied to the pixel electrode 74 to generate an electric field between the pixel electrode 74 and the common electrode 81, the linearly polarized transmission light that has passed the first polarizing plate 85 becomes elliptically polarized light in the liquid crystal layer 84 and passes through the second polarizing plate 86 to create white display.
Next, the shapes of the slits 76 and the protrusions 82 will be described. The slits 76 are formed by removing a part of the pixel electrode 74 by means of photolithography or the like, and the protrusions 82 are constituted by forming a resist made of acrylic resin or the like into a predetermined shape also by means of photolithography, for example.
The protrusions 82 are formed in a zigzag state across a plurality of pixels, and their linear portions are extended in the direction of a 45° angle with respect to the signal line 73 when viewed from the normal direction of the second substrate 78. At the approximately central portion of one pixel, a protrusion 82a extending from one adjacent pixel is bent to a 90° angle to further extend to the adjacent pixel, while a protrusion 82b extending from another adjacent pixel is arranged to lie parallel with the linear portion of the protrusion 82b that is bent at right angle, and made to reside near the corner portion of the pixel.
The slits 76 are formed so as to be positioned midway between a plurality of protrusions 82, and three slits 76 are formed in each pixel electrode 74 as shown in FIG. 6 in this example. Slits 76a are severally formed between the protrusion 82a and the protrusion 82b, and a slit 76b are formed between the protrusion 82a and the edge portion of the pixel electrode 74. The central lines of slits 76a are parallel with the adjacent protrusion 82 and in the direction of a 45° angle with respect to the signal line 73. The central line of the slits 76a corresponds to the extending direction of the slits 76a. Further, the extending direction of the slit 76b is similarly parallel with adjacent protrusion 82a. Note that the extending direction of the protrusion 82a adjacent to the slit 76b is bent at right angle in the pixel, so that the extending direction of the slit 76b is bent as well.
The liquid crystal molecules 84′ are inclined in the direction of a 90° angle with respect to the protrusions 82 and the slits 76, and inclined in opposite directions at the protrusions 82 and the slits 76. A pair of polarizing plates having cross-nicol arrangement is arranged outside a pair of glass substrates, whereby a 45° angle is made by the transmission axes of the polarizing plates and the direction of protrusions 82, while a 45° angle is made by the inclined liquid crystal molecules and the transmission axes of the polarizing plates when viewed from the normal direction of the polarizing plates. When the angle made by the inclined liquid crystal molecules and the transmission axes of the polarizing plates becomes 45°, transmission light can be obtained from the polarizing plates most efficiently.
The liquid crystal display panel having the above-described constitution is used for TVs and monitors where a wide viewing angle is particularly required.
On the other hand, traditionally, the demand for a wide viewing angle for liquid crystal display panels used in mobile devices such as cell phones or the like was not so high because the number of users for such devices was limited. However, demand for a wide viewing angle for the display section of liquid crystal display panels has been rapidly increasing in mobile devices that have recently become more functional.
Due to such increasing demand, the above-described MVA type of liquid crystal display panels has recently been developed in lieu of the conventional TN type that has been frequently been used for mobile devices.
Conventionally, among others, semi-transmissive liquid crystal display panels, which have the combined qualities of the transmissive type and the reflective type of LCD panels, have been developed in order to reduce power consumption requirements for liquid crystal display panels used in mobile devices intended for both outdoor and indoor use. The advanced state of development of the MVA type of liquid crystal display panels has been introduced in the semi-transmissive liquid crystal display panels disclosed in Japanese Laid-Open Patent Publication Nos. 2003-167253 and 2004-069767 and the like.
In developing the MVA type semi-transmissive liquid crystal display panel the inventors also initially experimented on the shapes of protrusions and slits formed in the conventional zigzag state as shown in FIG. 6, but noted that they were not suitable for small liquid crystal display panels for use in mobile devices. It is believed that this is due to the fact that the size of recently used pixels of small liquid crystal display panels producing high definition has been greatly reduced, while the zigzag-shaped protrusions and slits used in conventional medium and small sized liquid crystal display panels produced poor visual quality for TVs and monitors to which they were applied.
Then, the inventors conducted tests on the shape of a pixel electrode whose corners are chamfered as shown in FIGS. 13(b), (d), (f) and (h) of Japanese Laid-Open Patent Publication No. 2004-069767.
FIG. 8 shows a plane view of an example of a VA type of semi-transmissive liquid crystal display panel where the corners of the pixel electrode are chamfered while FIG. 9 shows the cross-sectional view taken along the IX-IX line of FIG. 8. Note that the reference numerals used in FIGS. 8 and 9 correspond to those used in FIGS. 6 and 7 and explanation therefor has been omitted.
Reference numeral 74a denotes a pixel electrode formed on the so-called light transmission section of a semi-transmissive display panel, and it is formed of ITO or such other transparent electrode material. The pixel electrode 74a is octagonal in shape with chamfered corner portions and resides almost entirely in the light transmission section. In the chamfered pixel electrode 74a, the distance from the center of the pixel electrode 74a to the end portion of a pixel electrode 74b is approximately the same in all directions.
Pixel electrode 74b is likewise composed of ITO and is formed on the so-called reflection section simultaneously with pixel electrode 74a. Reference numeral 74′ denotes a reflection electrode constituted for the purpose of reflecting outside light at the reflection section. The reflection electrode 74′ is made of a metallic material having high reflectivity such as aluminum. Note that the reflection electrode 74′ is formed on the lower layer of the pixel electrode 74b to perform conduction and function as an electrode as well, but it need not electrically conduct with the pixel electrode 74b nor function as an electrode. What is important is that the reflection electrode should be capable of reflecting outside light at the reflection section.
Reference numeral 82′ denotes a protrusion formed in the light transmission section. Unlike the linear protrusions formed in the entire pixel region, this protrusion is cross-shaped and independently resides only in the light transmission section. With such protrusion 82′ and the pixel electrode 74a whose corners are chamfered, it is possible to control the alignment of liquid crystal molecules evenly in the light transmission section of a small pixel in particular, and thereby produce a VA type of liquid crystal display panel with a wide viewing angle.
Reference numeral 82″ denotes an inverted letter Y-shaped protrusion formed to reside only in the reflection section. Note that reference numeral 90 formed on the second substrate 78 denotes a part that is provided to make the distance of outside light passing through the reflection section and the distance of light originating from the backlight passing through the light transmission section approximately the same, and is referred to as a topcoat.
Further, Reference numeral 91 denotes a contact hole, and a drain electrode 75′ of the TFT 75 is made to electrically contact the pixel electrode 74b via the contact hole 91.
Scanning signals are sequentially input by one frame to the scanning lines 72 shown in FIG. 8 and FIG. 9. However, a given level of voltage is applied to the scanning lines 72 even when there is no input of scanning signals
At this point, while the scanning lines 72 are in an exposed state (viewed on a plane as shown in FIG. 9), a potential difference occurs between the scanning lines 72 and the common electrode 81 at any time, and the liquid crystal molecules 84′ in this vicinity are always inclined.
As a countermeasure to picture quality deterioration, a known structure, is employed, the so-called Cs on-gate structure where the pixel electrode 74a is arranged on the scanning lines 72 in an overlaid manner. According to this structure, the end portion of the pixel electrode 74a is laid above the scanning line 72 to avoid affecting the quality of display, and the resulting potential difference that occurs between the scanning line 72 and the common electrode 81 does not affect the liquid crystal molecules 84′, while the potential difference that occurs between the pixel electrode 74a and the common electrode 81 serves to control the inclination of the liquid crystal molecules 84′.
However, even if this structure is employed in the liquid crystal display panel represented in FIGS. 8 and 9, the scanning line 72 appears completely at the position where the corner portion of the pixel electrode 74a is chamfered as illustrated in FIG. 10, which is a cross-sectional view of the liquid crystal display 70 taken along the X-X line of FIG. 8, while the potential difference that occurs between the scanning line 72 and the common electrode 81 causes the constant inclination of the liquid crystal molecules 84′, which results in the constant leakage of light originating from the backlight to constantly leak even during black display, thereby reducing contrast efficiency.